35 mm film is commonly used in cinematography, and several image formats for use with it are well known. These are defined by the aspect ratio of the image, namely the ratio of the width of the frame to its height. The frame height is normally slightly less than the frame pitch, which is the distance along the length of the film from the top of one frame to the top of the next (and is the same for all three formats). "Normal" 35 mm film uses frames with an aspect ratio of about 1.33. This matches the aspect ratio of a television picture. "Wide-angle" film uses a frame aspect ratio of about 1.85, so that for the same picture height on the projection screen, the wide-angle film format gives a wider picture than the normal film format. Accordingly, some of the image is lost when displayed on the television screen. On the film, the frame has the same width as for normal film but has reduced height. "Anamorphic" film provides an alternative format for producing a wide picture, and in this case the projected picture has an aspect ratio of about 2.35 to 1. The frame height on the film is nearly equal to the frame pitch. The width of the frame on the film, however, is very nearly equal to the frame width used in normal and wide-angle film. The resulting frame on the film has the lowest aspect ratio of the three formats, being about 1.2 to 1. However, cameras used to film in the anamorphic format include in their optical system a cylindrical lens element which has the effect of compressing the image in the horizontal direction, while leaving it unaffected in the vertical direction. The compression ratio is typically 2:1. Thus, a circle is recorded on the film as an ellipse with its major axis running vertically. Compensation for the horizontal compression of the image during filming is provided during optical projection by another cylindrical lens element, to produce a wide picture on the screen having a higher aspect ratio than either wide-angle or normal format film provides.
It may be required, at various times, to broadcast film in any of the three formats, and it is clearly desirable that the same telecine projector should be able to handle all three formats with the minimum of alteration or adjustment.
When a video signal is produced from wide-angle or anamorphic film stock, special steps have to be taken to produce a picture for display with the television aspect ratio of 1.33 to 1. For the most part this means that only part of the film picture is used, and the portion selected is panned across the film frame as required to follow the action in the scene. Part of the filmed image on one or both side edges of the picture will be lost, and in the case of anamorphic film this amounts to about half the total image width. [For certain parts of the film, e.g. the title and credits, it may be preferred to televise the whole width of the film picture with black bands at the top and bottom of the video picture, or to distort the video picture by horizontal compression to give a 1.33 aspect ratio without losing any picture information.]
The preferred embodiment of the invention, described below, is of the type using an image sensor comprising a charge coupled device (CCD). Charge coupled devices are a recent development in solid state semiconductor techniques and are well suited to image sensing roles.
Packets of charge may be created at various sites in the device (a semiconductor wafer) by incident light, and the rate of charge generation is linearly related to the incident light intensity. Accordingly, when an image is incident on the device, the image generates, at each site, a packet of charge whose size represents the image intensity at that site. The packets of charge may be moved from site to site through the device, to a suitable output terminal where they are read one by one.
In addition to photo-sensitive sites, the device will usually comprise a shift register formed by sites not sensitive to light. Packets of charge are transferred in parallel from the photosensitive sites to respective sites of the shift register, and are then shifted along the register to be read out in serial form at one end of the register. Each photo-sensitive site and the corresponding non-sensitive site form what is referred to below as a cell. The resulting electrical signal can be processed to produce a standard video signal representing the image falling on the charge coupled device.
It is the linear relation between light intensity and the rate of electron production, and the ease of moving electron packets through the device which make charge coupled devices well-suited to image sensing roles. However, three factors combine to make it difficult to design a telecine projector which uses a charge coupled device as a sensor and which can be used with all three film formats described above. Firstly, a large number of sensing points are required, especially with anamorphic film, to obtain a satisfactory resolution in the final television picture. Secondly, the time available for making an exposure and reading out the results is limited, being dependent on the time for which an individual frame is projected. Finally, there is a maximum rate at which signals can be read from the charge-coupled device. This rate is governed in part by the speed with which charge packets may efficiently be moved through the device, and primarily by the rate at which the output amplifier of the device can process signals.